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Vase Tunicate
Ciona intestinalis, also known as the vase tunicate, is a sea squirt of the family cionidae. The organisms are simple filter feeders and are typically sessile, attached to a marine surface, but have also been witnessed free swimming, The tunicate is solitary and has a tubelike, membranous body that can grow up to 20 cm in length (1), C. intestinalis is hermaphroditic but cannot self fertilize, utilizing broadcast spawning; emitting both eggs and sperm into the water column where they can survive for 1-2 days (1). The utility of the organism as a model species for development was discovered in the early 1900's. It is still used today in the fields of development and embryology, but has also found a home in genomics and phylogenetics as well. Despite its wide application, the remainder of this article will look at C. intestinalis ''in the context of genetics. ''Ciona intestinalis ''as a Model Organism ''C. intestinalis has been used as a model organism, specifically in development and embryology, due to its relative simplicity in the early stages of life. The C. intestinalis ''genome is also rather simplistic, having foregone the duplications seen in many vertebrates (2). Ascidians have simple embryos which are largely transparent, develop very quickly, and are easily genetically manipulated due to the oviparous nature. Even larval and adult stage ''C. intestinalis ''have relatively low cell counts, with tail-bud stage embryos possessing 1000 cells and mature swimming tadpoles having 2600 cells (2). Early researchers generated extremely detailed fate maps denoting the physical destinations of cells throughout development and the consequent tissue types (2). The simplistic nature of ''C. intestinalis made it an early favorite as a model organism for the study of development and embryology. The rigorous early research and continued use of C. intestinalis ''has allowed it to persist as a powerful and useful model organism today. Recent Work: Transiently Expressed Connexin Determines Neural Fate A recent genetic study done by Hackley ''et al ''used ''C. intestinalis ''to look at connexin expression and the role it plays during development. In a screen the group identified a mutation, later to be dubbed the ''frimousse ''line, that led to a severe disruption in neural plate development (3). During gastrulation the anterior most cells of the neural plate undergo FGF induction which directs them to differentiate into the cells that make up the neural crest and its derivatives (3). However, in the frimousse mutants after initial induction the cells revert back to the default tissue type; epidermis. Hackley's group was able to track the genetic mutation that is signature of the ''frimousse ''line to the connexin-11 gene, and shows the connexin-11 is transiently expressed during development corresponding to the formation of the neural plate (3). They then show evidence to support the notion that the connexin-11 gene may be modulated by calcium transients. To look at the relationship between calcium transients and and connexin-11 the group uses a gap junction inhibitor beta-glycyrrhetinic acid (BGA). BGA mimics the mutant by blocking gap junctions which would normally be at least partially constituted by connexin-11. The BGA treatment phenocopies the ''frimousse mutant and supports the groups claim (3). Overall, the claim that connexin-11 is key for proper neural plate development seems justified, Whether or not the group showed substantial evidence that BGA is an analog to the frimousse ''mutant line and calcium transients are responsible for modulating connexin-11 expression is up to the individual reader, but it seems there is more to be worked out. Early Work: Block to Self-Fertilization One of the more famous early ''C. intestinalis ''researchers was Thomas Hunt Morgan. ''Morgan had witnessed the block to self fertilization and compated it to that of a number of flowering plants. Following his curiosity, Morgan set out to determine the mechanism by which ''C. intestinalis ''can block self fertilization. Due to the age of the article the descriptions are rather brief but in the summer of 1923, Morgan was able to cause self fertilization by removing the egg from its surrounding membranes (4). Morgan squeezed the eggs from their membranes creating both whole and fragmented eggs in suspension (4). The eggs were then fertilized with sperm from the same individual and allowed to mature, Morgan noticed proper division through the third cleavage, but did not take careful note of the later stages. Because the sperm entered the fragmented egg and caused fertilization Morgan claimed the membrane was not responsible because the sperm must enter the egg to fertilize (4). He also goes to claim that it is not the follicle cells that sorround the egg because when removed eggs are still unable to be fertilized. Morgan does not go on to make any totally concrete conclusions but does state that the self fertilization block is likely a reaction between maternally deposited material and its own sperm. While not falling within the guidelines of today's scientific writing standards, Morgan's article is clearly written and shows scientific prowess. References 1. http://en.wikipedia.org/wiki/Ciona_intestinalis 2. Christiaen L, Wagner E, Shi W, Levine M. 2009. The sea squirt Ciona intestinalis. Cold Spring Harbor protocols 2009: pdb emo138 3. Hackley C, Mulholland E, Kim GJ, Newman-Smith E, Smith WC. 2013. A transiently expressed connexin is essential for anterior neural plate development in Ciona intestinalis. Development 140: 147-55 4. Morgan TH. 1923. Removal of the Block to Self-Fertilization in the Ascidian Ciona. Proceedings of the National Academy of Sciences of the United States of America 9: 170-1